CHICAGO – Casey Fien learned to throw his amazing cutter, the pitch that turned his career around, from a guy who works out of the bullpen in New York.

We know what you’re thinking. But it’s not him.

Still, it’s hard to imagine cutter virtuoso Mariano Rivera doing a better job of teaching Fien his out pitch than Ricky Bones, bullpen coach for the Mets. It was Bones, an 11-year major league pitcher, who took Fien aside during the winter of 2009, shortly after he got his first taste of the big leagues, and showed him that if he gripped the ball a little off to the side, added some extra pressure to his fingertips, and released the ball with his fingers straight over the top, it would appear to move sideways several inches as it approaches the plate.

“It’s a backdoor cutter — comes in hard on righties, cuts to the back corner of the plate to lefties. That’s a big pitch,” manager Ron Gardenhire said. “I just love the conversations I hear when he comes in from the bullpen — the catchers, they’re always [saying], ‘OK, let’s do what you do.’ I assume we’re talking about the cutter.”

And it’s the pitch that was missing when the Tigers gave him a shot in 2009. After a solid season at Class AAA Toledo, Detroit called him up for the stretch drive as the Tigers battled the Twins for the AL Central title. But hitters quickly figured out that he only had two pitches, a 95-mph fastball and a curveball that they could pick up right away.

“They were hunting fastballs, and I couldn’t get them off it with my curve,” said Fien, and a 7.94 ERA in nine appearances was the result. “So I needed something.”

He took that search with him to winter ball in Puerto Rico, where he met Bones, a 63-game winner over 11 seasons. The retiree had a suggestion: Try the cutter, which hitters wouldn’t be able to distinguish from his fastball.

“That’s when everything just started to click. I discovered I can throw it for strikes — and not just strikes, but quality strikes. Not down the middle, but hitting the corners,” Fien said. “The first couple of weeks, you never know where that ball’s going to go. But once you start playing catch and you see guys having trouble catching it, you know you’re doing something right.”

Still, it took a few years, and a few adventures, before anyone realized it. The Tigers let him become a free agent after 2010, and he signed with Houston, then injured his elbow just a couple of months into the season. When the Astros released him late in the 2011 season, he headed to Mexico, signing with a team in Guasave, on the Gulf of California about 350 miles south of the U.S. border.

“You realize that you must truly love baseball when you go down there,” Fien said, shaking his head. “Now I know how Latin guys feel when they come here. I didn’t know the language, had a hard time figuring out money. Even buying food was a challenge.”

And the baseball wasn’t exactly pitcher-friendly, either.

“Big boys that swing bats hard in small ballparks,” Fien said. “They want to see their home runs,” and ensure it by playing in tiny parks. One had a foul pole only 290 feet away; another was 350 feet to dead center.

“You definitely had to watch where you were putting the ball. You didn’t want to miss over the plate,” he said. “And if you don’t do well, hasta la vista, you’re done. The competition was cutthroat, because each team was only allowed five” non-Mexicans.

Still, Fien thrived with his cutter, and got noticeably better, he said, at hitting his spots precisely. When he went home for Christmas break, the Twins called with an offer to come to camp. He accepted, and when he was assigned uniform No. 85, used it as even more motivation.

He was one of the final cuts in the spring of 2012, then was called up in July. And he has thrived ever since, finishing 2012 with a 2.06 ERA, then gradually earning a job as one of Gardenhire’s most trusted setup men this season.

Fien has allowed the fewest hits per inning on the staff, the fewest walks per inning, too, and except for closer Glen Perkins, the most strikeouts. His 2.66 ERA is second to Perkins as well, and he’s allowed only five of 33 inherited runners to score, best rate in the bullpen.

“Having the cutter to go with my fastball made all the difference,” Fien said. “Yeah, I’m glad I found it.”